Write precompensation is a technique associated with the minimization or removal of the effects of nonlinear transition shift ("NLTS") that can occur in high density magnetic recording. Nonlinear transition shift is a write effect caused by magnetostatic interactions that occur between closely spaced magnetic transitions. When adjacent magnetic transitions are recorded closely together NLTS causes a transition that immediately follows a preceding transition to be shifted or drawn toward the preceding transition such that spacing on the media is altered from ideal. When uncorrected, NLTS causes serious degradation of overall recording performance.
An illustration of the effects of NLTS on the write waveform in a magnetic recording channel is given in commonly assigned copending U.S. patent application Ser. No. 08/215,686, filed Mar. 22, 1994, entitled "Apparatus and Method for Evaluating Nonlinearities in a Magnetic Recording Channel Using Pseudo-Random Sequences," the disclosure of which is hereby incorporated by reference in its entirety as if fully set forth herein, and in an article by Newby et al., entitled "The Effects of Nonlinear Distortion on Class IV Partial Response," IEEE Trans. Magn., Vol. 22, 1203-1205 (1986).
As magnetic recording densities become greater and greater write precompensation techniques have become increasingly important to compensate for the detrimental effects of NLTS. Write precompensation involves delaying the times at which adjacently recorded transitions are written onto a magnetic medium so that such adjacent transitions are recorded where intended, i.e., in proper bit spacing on the media relative to a write clocking signal. An example of a write precompensation circuit is disclosed in commonly assigned U.S. Pat. No. 5,341,249 issued Aug. 23, 1994, to Abbott et al., the disclosure of which is hereby incorporated by reference in its entirety as if fully set forth herein.
A write precompensation circuit "looks" at the user data stream as it is written to the disk, and detects the situation where two or more transitions immediately follow each other without intervening bit times. The write precompensation circuit is able to adjust the relative delay (or phase with respect to the write clock) of a transition following a preceding transition in order to carry out precompensation relative to the write clock signal. Application of precompensation delay causes the affected transitions to be time delayed by an appropriate amount, often expressed as a percentage of a nominal bit cell period established by the write clock signal. A write precompensation circuit may have eight programmable percentage delay increments, for example, to implement an appropriate precompensation delay.
Recently, sampled data detection techniques such as Partial Response ("PR") signaling and Maximum Likelihood ("ML") sequence detection (collectively "PRML") have been employed in magnetic recording systems. An example of a PRML data channel architecture is illustrated the above-mentioned U.S. Pat. No. 5,341,249 to Abbott et al. With the recent emergence of PRML systems in magnetic recording, nonlinear transition shift and write compensation therefore has become of particular concern. This is because a PRML recording system is typically modeled as a linear channel that fundamentally requires linear superposition principles to hold between recorded magnetic transitions to achieve optimum performance.
Representative samples of the state of the prior art with regard to the identification and measurement of NLTS in a PRML channel can be found in the aforementioned Newby et al. article and in Palmer et al., "Identification of Nonlinear Write Effects Using Pseudorandom Sequences," IEEE Trans. Magn., Vol. 23, 2377-2279 (1987); Ziperovich, "Performance Degradation of PRML Channels Due to Nonlinear Distortion," IEEE Trans. Magn., Vol. 27, No. 5, 4825-4827 (1991); and in U.S. application Ser. No. 08/215,686. The prior methods developed to measure NLTS outlined in these papers and patent application generally fall either the transform based approach or the correlation based approach. Additionally, spectrum analysis techniques have been used to measure the effects of NLTS.
Palmer et al. provides an example of the transform based approach using a high speed digital oscilloscope and a computer to identify nonlinear write effects. In Palmer et al., a pseudo-random sequence data pattern is written onto a computer disk, read back and captured by a digital oscilloscope. The digitized waveform is then processed in a digital computer through the use of Fourier transforms to obtain a channel transfer function. The inverse transform of the channel transfer function yields a time domain waveform in which various nonlinear effects manifest themselves as echoes, or small perturbations, that appear on either side of a main dipulse. This approach requires the use of expensive equipment and is not suited to a mass production environment because of the involved nature of the testing procedures and computations.
The correlation based approach disclosed in U.S. application Ser. No. 08/215,686 offers a simplified alternative to the transform based approach. In this prior application information regarding nonlinearities similar to that obtained from the Palmer et al. transform based approach may be obtained directly on an oscilloscope, for example, through the use of correlation techniques in the time domain. The amount of NLTS may be measured without the need for frequency domain transformation and manipulation, and without the need to import the digitized waveform into a computer for processing. Using this process, nonlinearities in the write waveform may be determined by writing a predetermined pseudo-random sequence waveform onto a magnetic recording medium, reading back the information and evaluating the autocorrelation of the readback sequence for a predetermined shift, M, where the nonlinearities of interest are known to occur. While the correlation based approach offers the advantages of a simplified test set-up and procedure, it is still not practical to optimize write precompensation for each storage device during manufacture by using the above-described techniques.
The spectrum analysis approach uses a spectrum analyzer and a digital computer to measure the harmonic content of a specific data pattern written and then read back from a magnetic recording medium. Like the transform based approach, expensive equipment is required and application to mass production is not practical.
After the amount of NLTS for a nominal read/write channel is measured using one of these three approaches, an estimated amount (based on the amount of NLTS) of write precompensation is applied to the write waveform and the test repeated to determine the amount of any remaining NLTS. The testing process may then be repeated until an amount of precompensation that minimizes NLTS is found.
While these prior approaches have been able to determine the amount of nonlinear transition shift and, through controlled experimentation, an approximate amount of write precompensation for a nominal read/write channel and head/media combination, individualized write precompensation optimization in a mass production environment has heretofore not been possible. Therefore, a need exists for a method adapted to mass production which is capable of determining a more optimal amount of write precompensation.
Accordingly, it would be desirable to provide a system capable of optimizing write precompensation on an individualized disk drive basis in a mass production environment to take e.g., variations in manufacturing processes and materials into account. It would also be desirable to provide a system capable of taking advantage of existing disk drive hardware and microprocessor capability to optimize write precompensation.